The invention concerns a method and an arrangement for producing castings.
In energy machine construction there is nowadays a trend to use stationary gas turbines with a high specific output as an alternative to the nuclear power stations which in many cases are rejected for environmental reasons, in which it was possible to manage with steam turbines. The higher working temperatures of gas turbines require the use of high-alloyed iron- and in particular nickel-based alloys which have considerable contents of Ti, Al, B, Nb, Ta, W etc in order to achieve the required properties. Hitherto gas turbines were preferably used as engines for aircraft, for which it was possible to manage with comparatively small turbine shafts. Relatively small ingots of diameters of 500 mm and below were required for the production thereof, which could be produced by means of re-melting methods with self-consumable electrodes, to afford an adequate quality standard. The term adequate quality standard is used in particular to denote a rough ingot which is substantially free from macroscopic textural and structural non-homogeneities such as segregation phenomena and other flaws which are known as xe2x80x98frecklesxe2x80x99 and xe2x80x98white spotsxe2x80x99.
White spots are flaw locations which are depleted in respect of alloying elements in comparison with the rest of the material. That flaw phenomenon is only known from the vacuum electric arc method with self-consumable electrodes, and it is assumed that this flaw phenomenon is caused by dendrite branches which drop down from the electrode tip and which are not fused in the molten sump. In the case of the electroslag re-melting method with self-consumable electrodes, that flaw phenomenon has hitherto not been observed.
Freckles are spot-shaped or fleck-shaped segregation phenomena which occur in isolated form and which can occur upon hardening of high-alloyed ingots along the dendrites if the alloy contains elements whose density differs considerably from the density of the basic alloy. Accordingly iron- or nickel-based alloys which contain high contents of specifically light elements such as for example Ti or Al but also specifically heavy elements such as W, Nb, Ta are particularly susceptible to that flaw phenomenon. While in the case of ingots of smaller dimensions of up to about 400 to 500 mm ingot diameters that flaw occurs only in isolation and only under unfavourable re-melting conditions, the production of flaw-free ingots of larger diameter is as good as impossible, even with the best control of the re-melting conditions. This is to be attributed to the fact that the long hardening times and large sump volumes which are inevitable when producing large re-melting ingots on the one hand result in a coarse hardening structure while on the other they promote segregation phenomena.
Now however the construction of stationary gas turbines with a sufficiently high specific output requires large turbine shafts, for the production of which correspondingly large rough ingots of diameters of substantially over 500 mm, preferably up to 1000 mm, are again required. In accordance with the present state of the art in regard to re-melting with self-consumable electrodes sufficiently flaw-free rough ingots cannot be produced from the alloys required for that purpose.
DE-196 14 182 C2 discloses a short, water-cooled, downwardly open chill mold for producing ingots or blocks, in which a casting level is covered by an electrically conductive slag in which the block or ingot is shaped in the lower part and withdrawn therefrom either by lifting the chill mold or by lowering the block or ingot. At least one current-conducting element which is not directly water-cooled is fitted into the wall of the chill mold which is formed from water-cooled elements, in such a way that the current-conducting element on the one hand comes into contact with the slag bath and on the other hand does not reach the level of the liquid metal; contact with a current source is made by way of that element which is disposed completely beneath the surface of the slag bath.
U.S. Pat. No. 5,799,721 discloses a method of re-melting in particular steels and Ni- and Co-base alloys to form an ingot by melting away at least one self-consumable electrode in an electrically conductive slag bath, wherein the ratio of the cross-sectional area of one or more consumable electrodes to the cross-sectional area of the ingot to be produced as the casting cross-section is selected to be greater than 0.5 and a melting rate in kg/h which corresponds to between 1.5 and 30 times the ingot diameter is set; the equivalent ingot diameter which deviates from a round cross-section is calculated from the periphery of the casting cross-section. In a funnel chill mold the melting rate in kg/h corresponds to between 5 and 15 times the equivalent ingot diameter calculated from the periphery of the casting cross-section and the ratio of the cross-sectional area or areas of the consumable electrode or electrodes to the cross-sectional area of the casting cross-section is equal to or greater than 1.0, wherein the ingot is shaped in the lower narrow part of the funnel chill mold and the slag bath extends into the enlarged upper part thereof.
In a method of producing castings of metal in accordance with GB-A-1 568 746 the ESR chill mold is provided with electrically insulated water-cooled current-conducting elements. Solid metal in the form of granules is continuously fed to the slag bath in order to result in the casting of a bloom of any length. Granules of that kind are of a higher density in the solid condition both than the slag bath and also the liquid metal of the molten sump, with the result that granules of that kind fall very quickly through the hot slag and do not or only very partially melt there, by virtue of the short residence time. If unmelted particles pass into the liquid metal sump, they also drop there by virtue of their higher density to the phase limit. There however they no longer melt as there is no longer a sufficient supply of heat in that region. With advancing hardening, particles of that kind are then enclosed in the unmelted condition in the form of foreign metal inclusions of their own specific kind in the hardening structure. As such unmelted particles are of a different structurexe2x80x94and thus involve different propertiesxe2x80x94from the melted metal, they are unwanted in melted ingots of which a uniform, fine and fault-free hardening structure is required.
It is an object of the present invention to provide a method for producing substantially segregation-free and in particular freckle-free castings of metal, in particular high-alloyed steels and Ni- and Co-based alloys of large dimension in accordance with the electroslag melting or casting method using a per se known short, current-conducting, water-cooled chill mold, in the wall of which current-conducting elements which are not directly water-cooled are fitted in electrically insulated relationship with the part of the chill mold, which forms the casting.
The foregoing object is attained by the teaching of the independent claim; the appendant claims set forth advantageous developments. The scope of the invention also embraces all combinations of at least two of the features disclosed in the description, the drawing and/or the claims.
In accordance with the invention a substantially segregation-free and freckle-free bloom whose cross-sectional area is at most 90% of the part of the chill mold forming the casting is arranged therein and, using a slag bath which is heated by the flow of current and which is disposed in the region of the current-conducting elements of the chill mold, by continually quantitatively controlledly pouring in liquid metal is connected to the supplied metal; the level of the slag in the chill mold is kept approximately constant by a relative movement between the chill mold and the bloom until the bloom is radially doubled in the desired lengthxe2x80x94that is to say is surrounded by a cast casing or jacket layerxe2x80x94; that operation can be repeated with the doubled bloom with a chill mold of larger dimension one or more times until the desired final dimension of the casting is reached. The method is basically suitable for any cross-sectional shapes. If however rough ingots which are subjected to further processing by forging are required, then round ingots are most appropriately produced.
When carrying out the method it is important that the casting or melting speed respectively is so set that the sump depth resulting therefrom permits upwardly directed, segregation-free hardening. It has proven to be desirable for the average casting or melting speed to be so set in kg/hour that it is between 0.25 and 5 times the sum of the equivalent bloom diameter and chill mold diameter in mm, wherein the equivalent diameter for shapes differing from round cross-sections is determined by the quotient periphery/xcfx80. In the case of extremely segregation-sensitive alloys, the best results are achieved if the casting or melting speed is set in the range of between 0.8 and 1.5 times the sum of the equivalent diameter corresponding to the above-indicated relationship.
The bloom required for carrying out the method is preferably produced by a re-melting method with a self-consumable electrode, in which case here a bloom dimension is selected, which ensures a fine-grain structure and with which the occurrence of freckles and segregation phenomena can be certain to be avoided. Basically the bloom can also be produced by an electroslag or other casting method as long as adequate freedom from freckles and segregation is guaranteed.
In the case of crack-sensitive alloysxe2x80x94and also for good and flaw-free bonding between the bloom and the doubled layerxe2x80x94it may be desirable to preheat the bloom to a temperature of up to 800xc2x0 C. For the production of homogeneous freckle-free and segregation-free ingots and castings for the production of forged components or the like the bloom is doubled with an alloy which is of the same chemical composition as the bloom. For particular purposes of usexe2x80x94for example the production of composite rollers which must have a tough core and a wear-resistant surfacexe2x80x94the bloom can also be doubled with an alloy of completely different composition.
For the purposes of carrying out the method it is necessary that the liquid slag bath is always disposed at the height of the current-conducting elements which are installed in the wall of the chill mold and which are not directly water-cooled and which are electrically insulated with respect to the rest of the chill mold, as it is by way of those elements that current can be fed into the slag bath. The return of the current is then effected by way of the bloom or the bottom plate on which the bloom rests. Due to the flow of current therethrough, the slag bath is maintained in a liquid condition and heated to such an extent that on the one hand the metal of the bloom begins to melt at its surface and on the other hand, even when liquid metal is slowly poured in, for the purposes of the doubling effect, premature hardening is avoided at the location where the meniscus of the liquid level of metal is in contact with the water-cooled wall of the chill mold.
For starting the procedure the prepared bloom is put on to a stool which fits into the internal opening of the chill mold and which can either be water-cooledxe2x80x94and thus re-usablexe2x80x94or which can also comprise the same material as the bloom. The stool with the bloom carried thereon is firstly positioned in the chill mold in such a way that its upper edge just terminates with the upper edge of the lower, water-cooled part of the chill mold which forms the new surface of the bloom. Voltage is now applied, in which case however initially still no current flows as there is no conducting connection between the current-conducting elements and the bloom or the bottom plate. Then a pre-melted slag of the desired composition is poured into the gap between the bloom and the wall of the chill mold, whereby a current begins to flow as soon as the level of the slag passes into the region of the current-conducting elements in the wall of the chill mold. Now the desired electrical power is set to correspond to the dimensions of the bloom and the chill mold and after a short time which is sufficient to initiate melting of the surface of the bloom which is exposed to the slag bath, the operation of pouring in the metal which is to form the doubled layer is begun. In order always to keep the level of the slag in the region of the current-conducting elements, depending on the configuration of the installationxe2x80x94for example with a stationary stoolxe2x80x94the chill mold is raised continuously or in steps approximately as the level of the slag rises due to the supply of metal. If in contrast the installation has a stationary chill mold the stool is withdrawn from the chill mold in a corresponding fashion in order in that way once again to ensure an approximately constant level for the slag in relation to the current-conducting elements. In this respect the operation of pouring in liquid metal can be effected continuously or discontinuously in steps, according to the desired speed of formation. When using a stepwise supply however the volume of the amount of metal in an individual step is not to exceed the volume of the slag bath. Both when using a continuous and also a stepwise supply of metal however care is to be taken to ensure that the above-mentioned mean casting rate is not exceeded.
The lifting movement of the chill mold or the lowering movement of the bottom plate with the stool can be effected in per se known manner again continuously or in steps, in which respect the mean lifting or withdrawal speed must again be suitably matched to the metal supply speed. When adopting a stepwise mode of operation, it is to be noted that the individual step is not to be greater than the height of the current-conducting elements which are installed in the wall of the chill mold. Each lifting step is followed by a pause until the level of the slag has again approximately reached the original level. When using a stepwise mode of operation, a return stroke can further be incorporated between the withdrawal step and the pause, in which case then the withdrawal stroke, the return stroke and the pause must be so matched to each other that they correspond to the mean metal supply speed.
If in contrast operation is implemented with a continuous withdrawal speed, it may be helpful, to afford a good surface, if the chill mold performs an oscillating movement, as is known from continuous casting.
It is to be considered as a further essential feature that the casting which rests on a lowerable bottom plate is withdrawn from the fixedly installed chill mold in such a way that the level of the slag bath in the chill mold remains approximately constant. In addition the operation of pouring in the liquid metal or melting the added solid metal pieces is preferably effected under a protective gas atmosphere of controlled composition and controlled pressure; the controlled reduced pressure is set in the range of between 1 and 600 mbar, while in the case of an increased pressure a value of more than two bars is preferred.
Instead of pouring in liquid metal, solid metal can also be introduced into the slag bath in the form of bars, chips or turnings or granules and caused to melt therein. The supply of metal into the gap between the bloom and the chill mold is continued in the above-described manner until the entire bloom has been doubled thereon. Then the supply of energy to the slag bath is switched off and the doubled bloom is removed from the installation, after complete hardening of the doubled layer.
In principle the method can be carried out in the open air as the liquid slag bath protects the level of metal therebeneath, from the oxygen in the air. For the production of high-grade alloys however it is recommended that the method be carried out under a controlled protective gas atmosphere, in which case it is also possible to operate under a reduced pressure or an increased pressure, depending on the respective demands involved.
In an arrangement according to the invention for producing castings with a low degree of segregation and in particular a low degree of freckling of metalxe2x80x94in particular steels and Ni- and Co-based alloys in accordance with an electoslag melting or casting methodxe2x80x94with a short, current-conducting, water-cooled chill mold, in the wall of which current-conducting elements which are not directly water-cooled are installed in electrically insulated relationship with the part of the chill mold which forms the casting, and which is provided with a bottom plate associated downwardly with the chill mold, for carrying out the described method, a casting gap for receiving liquid metal is delimited by a bloom placed on the bottom plate and the chill mold. Two chill molds which are connected in succession are preferred; in that case the inside diameter of the subsequently connected chill mold should be larger than that of the preceding chill mold.